1. Field of the Invention
The present invention relates to a method of fabricating a silicon single crystal wafer used for fabricating a semiconductor device or the like and to a silicon single crystal wafer. More particularly, the invention relates to a method of fabricating a silicon single crystal wafer from a nitrogen-doped silicon single crystal grown according to the Czochralski method and to a silicon single crystal wafer fabricated by the method.
2. Description of the Related Art
The most common silicon single crystal wafer used for fabricating a semiconductor device is a one obtained by processing a single crystal ingot grown according to the Czochralski method (CZ method).
According to the CZ method, a single crystal is grown by dipping a seed crystal in a silicon molten in a crystal crucible, pulling the seed crystal away from the molten while rotating the quartz crucible and the seed crystal to grow a cylindrical silicon single crystal, thereby developing an ingot.
In a wafer obtained from the silicon single crystal grown by the CZ method, however, various grown-in crystal defects occur.
One of the grown-in crystal defects is called an oxidation induced stacking fault (OSF) which occurs in a ring shape due to a thermal oxidation process. Since the OSFs occur in a ring shape, this region is called an OSF ring. The width of the OSF ring is usually about a few mm to ten mm. The OSF deteriorates a junction leak characteristic as one of semiconductor device characteristics.
Since oxygen precipitation does not easily occur in the region of the OSF ring, it is difficult to sufficiently form oxygen precipitation bulk micro defects (hereinbelow, called BMD) functioning as a gettering site of a heavy metal which is generated in a semiconductor device fabricating process, that is, IG (Intrinsic Gettering).
Since the OSF ring moves toward the peripheral side of the single crystal as the pulling rate increases, to form the OSF ring in the outermost periphery of a crystal, high-speed pulling of 1.0 mm/min or higher is performed.
However, voids defect called COP (Crystal Originated Particles) exist on the inside of the OSF ring. If nothing is done for the defects, an oxide film resistance characteristic and a junction leak characteristic of a semiconductor device deteriorate.
Consequently, a method of reducing void defects by performing heat treatment in a gaseous hydrogen or argon gas atmosphere is employed.
On the other hand, a method of reducing a void defect region by decreasing the pulling speed to 0.5 mm/min or lower to form the OSF ring in the center of a wafer is also proposed. According to the method, void defects do not occur in the outer side of the OSF ring, and an oxide film resistance characteristic is also excellent.
In this case, however, dislocation clusters often occur in the peripheral portion in a wafer. The dislocation clusters also deteriorate the junction leak characteristic. Further, in this region, oxygen precipitation does not easily occur, so that the IG function deteriorates.
To solve the technical problems, for example, Japanese Unexamined Patent Application No. 8-330316 discloses a technique capable of forming a void-free region in the whole area in the radial direction of a single crystal on the basis of the knowledge that a region in which an infrared scattering defect (void), OSF ring, and dislocation cluster occur can be specified by a ratio expressed by V/G where V denotes a rate (mm/min) of pulling a single crystal and G (° C./mm) denotes an average value of temperature gradient in a crystal in the pulling axis direction in a high temperature range from a silicon melting point to 1300° C.
Specifically, the publication discloses a technique capable of specifying a region including no grow-in crystal defect formed between an OSF ring and a dislocation cluster occurrence region by the V/G value and obtaining a silicon single crystal wafer in which a defect-free region is formed in the whole face by controlling the V/G value in the crystal axis direction and the radial direction at the time of growing a crystal to 0.20 to 0.22 mm2/° C.·min.
However, in a heat-treated wafer obtained from a single crystal grown by a high-speed pulling method, even after the heat treatment, micro void defects each having a size of 0.1 μm or less tend to remain. Consequently, in order to use a wafer for a device of a finder design pattern, such a micro void defect has to be dissipated by performing heat treatment for long time at high temperature.
In the case of pulling a single crystal having a larger diameter of 300 mm or larger, it is difficult to pull the single crystal at high speed. The pulling rate has to be regulated to an intermediate rate of 0.5 to 1.0 mm/min at which an OSF ring remains.
On the other hand, in the case of growing a defect-free region at low speed, it is very difficult to control the V/G value in a narrow range in both of the axial and radial directions of a single crystal. Moreover, oxygen precipitation easily occurs, so that it is necessary to add the IG function or the like by another means. Due to low speed, decrease in productivity is also caused.
To deal with such problems, recently, a method of doping a single crystal with nitrogen at the time of pulling the silicon single crystal in accordance with the CZ method is being variously studied.
For example, it is reported by H. Tamatsuka et al., “DEFECT IN SILICON III, PV99-1” p. 456 that by doping a single crystal with nitrogen, the size of avoid defect is reduced. As a result, by a high-temperature heat treatment in a gaseous hydrogen or argon gas atmosphere, the void defect is easily dissipated. A surface layer portion without a void defect from the surface of the wafer to the depth of 10 μm or more is formed.
However, in order to sufficiently and reliably form such a surface layer portion, oxygen concentration in the crystal has to be suppressed. In this case, it becomes difficult to form BMDs functioning as a getter site at a density of 1×109/cm3 or higher.
Consequently, only by a single crystal pulled at a high speed of 1.8 mm/min under an oxygen concentration condition of a very narrow range, both the front layer portion and the BMD can be formed. It cannot be said that the method is sufficient as an industrial wafer fabricating method, and there is a room to improve.
It is reported by M. Iida et al., “DEFECT IN SILICON III, PV99-1” p. 499 that, by doping a single crystal with nitrogen, a defect-free region is expanded, that is, the range of the V/G value of the defect-free region is widened. Simultaneously, a region in which the OSF ring occurs also expands. However, the OSF ring can be made disappear by lowering the oxygen concentration and, by the amount, the V/G value shifts to a larger value and the defect-free region expands.
As described above, by doping a single crystal with nitrogen, it becomes easier to form a defect-free region. However, it is still difficult to form a sufficient amount of BMDs in the defect-free region. Particularly, it is difficult to form BMDs in a wafer of low oxygen concentration.